In the past, a gas analysis apparatus has been used to measure a composition of exhaust gas emitted from an internal combustion engine of an automobile, such as NOx, and in recent years, a composition such as ammonia has also been measured.
As specific examples of measuring the gas composition such as ammonia, which has not been measured in the past, include a scene of research and development of a urea SCR (Selective Catalytic Reduction) system that can highly efficiently drive a diesel engine and also suppress a production amount of NOX, and other scenes. To specifically describe the urea SCR system, the urea SCR system is configured to, by spraying urea into high-temperature exhaust gas emitted from a diesel engine, and as a reducing agent, supplying ammonia produced by pyrolysis of the urea to an SCR catalyst, reduce NOX in the exhaust gas to change the NOX to harmless N2 and H2O.
In the case where in such a urea SCR system, an excessive amount of urea is supplied, ammonia is contained in an exhaust gas emitted from a muffler or the like to give rise to a bad odor, or to failure to meet environmental standards. For this reason, in order to know whether or not an adequate amount of urea can be supplied in various driving conditions, ammonia in exhaust gas is measured.
For example, JP-A2010-223650 discloses a gas analysis apparatus 100A that measures an amount of urea remaining in exhaust gas only with ammonia sensors 21A that can measure the concentration of ammonia. More specifically, as illustrated in FIG. 6, the analysis apparatus 100A is one provided with: a sampling pipe 1A of which one end is opened just before an SCR catalyst 94 and the other ends (of sampling pipe branches) are provided with the ammonia sensors 21A; and heaters 31A and 32A that are respectively provided so as to surround the sampling pipe branches 1A on upstream sides of the ammonia sensors 21A. Also, the gas analysis apparatus 100A is configured to measure first concentration of ammonia while heating the inside of one of the sampling pipe branches 1A with the heater 31A so as to keep the urea contained in the exhaust gas at a temperature lower than a pyrolysis starting temperature of the urea, and second concentration of ammonia while heating the inside of the other sampling pipe branch 1A with the heater 32A to temperature higher than the pyrolysis starting temperature of the urea, and from a difference between the concentrations, calculate an amount of the remaining urea contained in the exhaust gas.
In addition, it is expected that isocyanic acid, which may be produced in the case of introducing urea into the exhaust gas for the urea SCR, also becomes increasingly important in measurement in future because an influence of isocyanic acid on an environment, health, and the like is quantitatively evaluated. Further, JP-A2010-223650 also describes a method for calculating an amount of isocyanic acid produced from urea by a decomposition reaction.
Meanwhile, even in the gas analysis apparatus as disclosed in JP-A2010-223650, it is required to further improve measurement accuracy and reliability of the remaining urea and isocyanic acid in the exhaust gas.
Specifically, in order to accurately measure an amount of the remaining urea in the exhaust gas, it is necessary to completely decompose the remaining urea into the ammonia in the sampling pipe, and obtain an accurate amount of the ammonia derived from the remaining urea. However, at present, there is a possibility that sufficiently heating and completely decomposing urea in a misty state or powdery urea resulting from extracting water are not sufficiently performed.
On the other hand, in the case of measuring the amount of the isocyanic acid produced from the urea by heat of the exhaust gas, if even part of the remaining urea is pyrolyzed in the sampling pipe, in addition to the isocyanic acid produced by the heat of the exhaust gas in the first place, isocyanic acid newly produced in the sampling pipe is included. That is, the evaluation may be made on the basis of a more excessive amount than the amount of the isocyanic acid emitted from an actual vehicle.
In other words, even in any of the measurement of the urea and the measurement of the isocyanic acid produced from the urea, in order to increase accuracy and reliability, it is necessary to appropriately control the decomposition reaction of the remaining urea contained in mixed gas.
Also, in the case where the powdery urea arrives at each of the ammonia sensors without being decomposed, there exists another problem that the urea is attached on a surface of a sensor part, or the like to reduce sensitivity and reduce reliability of a measured value, and consequently calculated values of the urea and isocyanic acid amounts may become unreliable.